Flip chip assembly is a method for interconnecting semiconductor devices, such as integrated circuit (IC) chips, and microelectromechanical systems (MEMS), to external circuitry, such as circuit boards or another chip. Typically, solder bumps are deposited on pads on the IC chips on the top side of the wafer during the final wafer processing step. The circuit board for interconnection has pads configured to align with the pads on the IC chip. In order to mount the IC chip to the external circuit, the IC chip is flipped over so that its top side faces down. The IC chip is then aligned so that its pads align with matching upward-facing pads on the external circuit. The solder of the bumps is then caused to flow and subsequently solidify to join the respective pads of the IC chip and the external circuitry. The IC chip that is designed to be flipped over in this manner is referred to as a flip chip.
A circuit board that is configured to receive a flip chip includes a dielectric substrate on which are defined contact pads for receiving the solder bumps disposed on the IC chip pads. A solder mask, in the form of a patterned layer of material that tends to resist the flow of solder, is defined over the contact pads and exposed substrate material. The solder mask layer is patterned to contain the solder generally to the contact pads. The solder mask thus prevents undesirable spread of the solder beyond the contact pad and the immediately adjacent portion of transmission lines in contact with the contact pad. However, in high frequency radio frequency (RF) applications, the solder mask degrades signal transmission in the circuit line in the area near the contact pad.
As pitch (the space between trace lines) becomes smaller due to the increased density of IC chips and the increased number of interconnections, the use of solder masks becomes problematic. Controlling the size of the holes in the solder mask and registration of the holes to the metal circuitry become more important. The size of the holes in the solder mask layer may need to be larger than desired to account for the tolerances on the solder mask hole size and registration. Larger solder pads and variation in pad size affect the height of the solder joints. The solder joint height sets the gap between the chip and the circuit board. For reliability, the gap between the chip and circuit board is typically filled with an encapsulant adhesive, called an underfill. A larger and more consistent gap between the chip and the circuit board is desirable to facilitate the underfill process. The thickness of the solder mask also reduces the gap between the chip and the circuit board. Eliminating the solder mask also increases the gap through which the underfill will flow.
Furthermore, in RF applications, particularly high-frequency RF applications, additional restrictions on the design of interconnect structures and IC chip interconnections may be needed to avoid RF transmission losses.